1. Field of the Invention
This invention relates to molding compositions useful as encapsulants for electronic components and more particularly to epoxy resin type encapsulants.
2. Description of the Prior Art
It has been heretofore found desirable to encase electronic components in plastic compositions to provide an electrically insulative, protective and moisture resistant package. For example, semiconductor components, such as crystal chips which are provided with printed circuits bonded to lead wires, have been encapsulated with thermosetting plastic molding compositions. One such type of molding composition is based upon silicone rubber; however, encapsulants formed therefrom, while exhibiting the requisite dielectric insulation properties, are somewhat sensitive to moisture and for certain applications provide inadequate moisture resistance in terms of mechanical properties, such as linear expansion coefficient, and water vapor transmission rate.
In U.S. Pat. No. 3,789,038 issued Jan. 29, 1974 to the present inventors together with Curtis and O'Neill, there are described polyanhydride-cured epoxy molding compounds. Such compounds comprise an epoxide compound, a hardener which is either a polyanhydride reaction product of at least one alkyl styrene monomer and at least one maleic monomer or a prepolymer of such polyanhydrides, a catalyst and filler. Exemplary of such hardeners is the polyanhydride of alpha-methyl styrene and maleic anhydride (sometimes referred to as AMS/MA hardeners). Such epoxy molding compounds exhibit excellent moisture resistance in terms of water vapor transmission rate and linear coefficient of expansion, but for certain purposes their wet electrical properties are inadequate for use as semi-conductor encapsulants. For example, compared with commercial silicone-type encapsulants, AMS/MA hardened epoxies exhibit superior moisture resistance (in terms of change in linear expansion coefficient and water vapor transmission) but somewhat inferior high temperature wet electrical properties (such as dielectric constant and dissipation factor after exposure to a pressure cooker). It would be desirable if an encapsulant molding compound were provided which has excellent moisture resistance as well as wet electrical properties.